Tabular silver halide grains which contain parallel twinned crystal planes (referred to below as tabular grains) have the advantages indicated below in terms of photographic characteristics, and it is for this reason that they have been used in commercial high speed photosensitive materials in the past.
1) They have a large specific surface area so that a large amount of sensitizing dye can be adsorbed on the surface and they have a high minus-blue/blue speed.
2) The grains are arranged parallel to the base surface when emulsions which contain tabular grains are coated and dried so that it is possible to reduce the thickness of the coated layer, thereby increasing sharpness.
3) When, with X-ray films, a sensitizing dye is added to the tabular grains, the extinction coefficient of the dye is greater than the extinction coefficient for the indirect transition of the silver halide and it is possible to achieve a marked reduction in cross-over light in this way, thereby preventing any worsening of picture quality.
4) A high covering power can be realized when developing tabular grains which have a high aspect ratio, the silver density and dye density are evened out, and there is an improvement in terms of the RMS granularity characteristics.
5) The absorption of radiation increases exponentially with respect to the thickness of the grain, but with tabular grains the grains are thin and so the amount of radiation absorbed per grain is low and there is little fogging due to natural radiation with the passage of time.
6) There is little light scattering and it is possible to obtain images which have a high resolution.
7) The grains have flat, parallel surfaces giving rise to an optical interference effect with respect to the parallel plates and it is possible to increase the light utilization efficiency by making use of this effect.
8) The rate of development is proportional to the specific surface area of a silver halide grain, and tabular grains have a large specific area and, therefore, a high development rate.
Tabular grains have been widely used in high speed sensitive materials in the past because of their many advantages, such as those indicated above.
The term "aspect ratio" as used herein signifies the ratio of the diameter to the thickness of the tabular grain. Moreover, the diameter of the tabular grain signifies the diameter of a circle which has the same area as the projected area of the basal plane of the grain when the emulsion is observed using a microscope or an electron microscope.
On the other hand, it is known that the various photographic effects indicated below can be realized by including iodide ions in the silver halide.
Thus, in connection with the photosensitive process:
(1) In the photosensitive process, the absorbing band wavelength is extended on the long wavelength side in the intrinsic absorption region, the extinction coefficient is increased, and the blue absorption efficiency is raised.
(2) The refractive index is increased and so a larger optical interference effect in the tabular grains can be anticipated, as described in Research Disclosure, 25330 (May, 1985).
(3) In the band structure, the valency electron band is raised in the Photoholes which are generated by the absorption of light accumulate in the parts which have a high iodide content. Thus the separation of electrons and photoholes is promoted.
Reference can be made to the disclosures made in JP-A-60-143331 and JP-A-60-143332, Journal of Imaging Science, 29, 193 (1985) and JP-A-63-92942 in connection with this effect in the case of grains which have a double structure. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
(4) When sensitizing dyes are adsorbed on silver halide layers which have a high iodide content and the grains are given a minus blue exposure, the iodide has the effect of increasing the photohole implanation efficiency from the sensitizing dye to the silver halide grains, and reactions occur with reduced silver nuclei within the grains with the release of electrons.
Reference can be made to the disclosures of Japanese Patent Application No. 62-251377 in connection with this effect.
(5) The iodide ions themselves have a photohole trapping effect.
(6) Lattice irregularity defects and dislocations occur between layers which have very different iodide contents and photographic effects arise on the bases of these defects and dislocations. Reference can be made to the disclosures made by J.W. Mitchell in Nippon Shasshin Gakkaishi (Journal of Japan Photographic Academy), volume 48, 191 (1985), and JP-A-63-220238.
On the other hand, in connection with the development process:
(7) The spread of the silver filaments after development is small and graininess is good. Furthermore, in the case of color development the spread of the dye cloud which is formed surrounding the grains is suppressed to a low level and this has the effect of improving graininess.
(8) The edge effect, due to the development inhibiting action of the iodide ion which is released in the transverse direction during development, is increased and this has the effect of improving resolution.
Furthermore, the iodide ion which is released in the longitudinal direction during development has an inter-layer development inhibiting effect which inhibits the development of adjacent layers.
(9) In cases where graininess is improved by stopping development without developing the whole of each grain and suppressing the spread of the filamental silver or dye cloud to a low level, or in cases where, in the course of a parallel development process such as color negative development, there is a DIR effect which is effective in the later stages of the development process, the later stages of development are suitably slowed down and the control can be achieved easily.
(10) There is an effect on the contrast enhancing development with glutaraldehyde which is often used with X-ray film systems.
Furthermore, the introduction of iodide ion has an effect on pressure characteristics by hardening the silver halide grains, as disclosed by P.V.McD. Clark and J.W. Mitchell, J. Phot. Sci., Volume 4, 1 (1956).
(11) The iodide promotes the adsorption of sensitizing dyes and additives on the silver halide grain surface.
Hence, the development of grains in which the effects of the aforementioned tabular grains and the effects of iodide ion as indicated above are combined is desirable.
In general, tabular grains have two or more parallel twinned crystal planes. It is necessary to have at least two parallel twinned crystal planes to form a tabular grain, but in grains which have three or more twinned crystal planes the speed of the inner part is increased and this is undesirable. This is because twinned crystal planes are a type of crystal defect and the speed of the inner part is increased by a synergistic effect when numerous twinned crystal planes are present. Hence, grains which have only two twinned crystal planes are the most desirable.
The shape of the main surface of a tabular grain which has only two parallel twinned crystal planes is hexagonal with an adjacent side ratio (length of the longest side)/(length of the shortest side) of from 2 to 1. In cases where the tabular grains are arranged on a flat surface with the closest packing with the basal planes parallel, configurationally, the shape of the basal planes which provides the best resolution and more or less equal resolution in all directions is the a hexagonal shape and hence this is the ideal sensor arrangement.
Reference can be made to the descriptions given in chapter 1 of Image Science, by J.C. Dainty and R. Shaw, published by the Academic Press Inc., London, 1974, in this connection.
From this point of view also, hexagonal tabular grains are the most desirable.
According to J.E. Maskasky, J. Imaging Sci., volume 31, 15-26 (1987), triangular tabular grains are grains which have three parallel twinned crystal planes. In this case, when comparing hexagonal tabular grains with triangular tabular grains which have the same projected area, the maximum diameter of a triangular grain is 1.23 times larger than the maximum diameter of a hexagonal grain, and so graininess is worse in the case of triangular tabular grains. Hence, triangular tabular grains are undesirable.
Furthermore, when dopants such as metal ions and changes in halogen composition are introduced to control the grains in an intended location, with a hexagonal tabular grain it is possible to introduce the dopant or change for control at the intended location since the six sides have more or less the same growth rate, and it possible in this way to obtain the intended photographic characteristics. Hence, hexagonal tabular grains are also preferred from this point of view.
Furthermore, an even size distribution of tabular grains is preferred. Thus, disadvantages such as those indicated below arise in cases where the size distribution is not mono-disperse.
1) High contrast (which is to say high gamma) characteristic curves cannot be anticipated.
2) A multi-layer system obtained by coating a mono-disperse large sized grain layer as an upper layer and a mono-disperse small sized grain layer as a lower layer provides a higher speed in terms of the utilization efficiency of the light than a coated emulsion layer in which large and small sized grains are mixed together, and the multi-layer effect cannot be utilized satisfactorily.
Here, a case which does not have good mono-dispersivity signifies (1) the admixture of rod like grains, tetrapod like grains, and grains which have a single twinned crystal plane or non-parallel twinned crystal planes with the tabular grains, (2) the admixture of triangular tabular grains, trapezoidal tabular grains and rhomboidal tabular grains other than hexagonal tabular grains with the tabular grains, and (3) tabular grain which have a wide projected grain size distribution.
On the basis of the facts outlined above, tabular grains which have a high iodide content, and which have only two parallel twinned crystal planes (and of which the shape of the basal plane is therefore hexagonal), and a large specific surface area, and which have a good mono-dispersivity are clearly desirable.
However, the main problem when iodide ions are introduced is that, as disclosed in JP-A-58-113928, a great many thick, non-tabular, grains are admixed with the tabular grains when a high iodide ion content is introduced into the central portions of tabular grains.
For example, the methods for the preparation of tabular grains containing a central portion of a high iodide content described, for example, by C.R. Berry and S.J. Marino, in Journal of Physical Chemistry, 62, 881 (1958), A.P.H. Trivelli and W.F. Smith, The Photographic Journal, 80, 285 (1940), E.B. Gutoff, Photographic Science and Engineering, 14, 248-257 (1970) and Cugnac and Chateau, Science et Industrie Photographique, 33, 121 (1962) all provide a high proportion of thick, non-tabular, grains and a wide grain size distribution, and they cannot be said to provide the distinguishing features of the tabular grains described above.
There is also a second problem in that with the conventional methods of grain formation (especially the methods in which iodide ion is added to the reactor before the introduction of the silver salt and the halide, and the methods in which silver iodide is used as seed crystals, as disclosed in U.S. Pat. Nos. 4,150,944, 4,184,877 and 4,184,878), it is impossible to provide a prescribed composition of fixed iodide content in the central portion of the tabular grain or to form a silver bromoiodide layer of uniform composition. Furthermore, with the methods in which silver iodide is used for the seed crystals, the proportion of hexagonal tabular grains among the tabular grains which are formed is low, and the proportion of deformed tabular grains, such as trapezoidal and rhomboidal tabular grains, is high.
Conditions of growth for providing tabular grains with a more mono-disperse grain size distribution have been suggested by the present inventors in JP-A-55-142329.
In this case, the coefficient of variation at an average grain diameter of 0.96 .mu.m was 11.6%, and the grain size distribution was very uniform for an emulsion consisting of multi-twinned crystal grains (grains which had a double structure with a silver bromoiodide layer which had a high iodine content for the core part), but the proportion of non-parallel twinned crystal grains was high because inappropriate nuclei forming conditions were used when forming the seed crystals.
Furthermore, the double structure, twinned crystal grains described in the illustrative examples of JP-A-60-143331 were prepared using a rush addition single jet method for nuclei formation and so the grains obtained had a low proportion of hexagonal tabular grains.
Various investigations have been carried out in connection with these problems.
For example, in connection with the first problem, as suggested in JP-A-58-113928, tabular core grains can be formed in the region where the proportion of formed non-tabular grains is low by having the state in the reactor before the introduction of the silver salt and the bromide salt essentially iodide ion free (iodide ion concentration less than 0.5 mol%), adjusting the pBr value to within the range of from 0.6 to 1.6, and by using essentially silver bromide (the iodide ion content of silver bromoiodide being preferably less than 5 mol% and most desirably less than 3 mol%), after which a high iodide content layer (as an intermediate with an iodide content preferably of almost the solid solution limit, more preferably of from about 6 to 20 mol%) can be deposited over the core grains, and then a layer of silver bromoiodide which has a low iodide content can be deposited over the top of this as a shell to form silver halide grains which have a triple structure.
However, in this case, only tabular grains which have a low iodide content in the central portion are obtained. Further, the intentional preparation of the parallel double twinned crystal grains has not been approached in this case.
The central portion also has a low iodide content when silver iodobromide tabular grains are formed using the methods disclosed in JP-A-59-99433, JP-A-61-14630 and JP-A-58-211143.
A method for the formation of tabular grains with a low proportion of non-tabular twinned crystal grains by limiting the iodide ion concentration in the reactor prior to the introduction of the silver salt and bromide in accordance with the values indicated below has been proposed in JP-A-62-151840, ##EQU1##
However, since the halide being added is the substantially bromide, the average iodide content of the nuclei grains in the illustrative examples is from 5 to 6 mol% or less in the silver bromoiodide, and the central portion still has a low iodide content. Further in this case, since most of the iodide ion used in the nuclei formation are previously present in the reactor, the silver iodide nuclei are first formed.
Furthermore, the mono-disperse twinned crystal grains in JP-A-51-39027 and JP-A-61-112142 are prepared by adding a silver halide solvent after nuclei formation, ripening the emulsion, and then growing the grains, but in both cases the grains are tabular grains of which the central portion has a low iodide content.
Furthermore, JP-A-63-151618 and Japanese Patent Application No. 62-319740 by the present inventors disclose monodisperse parallel double twinned crystal tabular grains. However, the grains used in their illustrative examples are tabular grains of which the central portion has a low iodide content (7 mol% or less).
However, tabular grains which have a low iodide content silver bromide in the central portion and a layer which has a high iodide content on the outside have the following disadvantages:
a) A large difference in iodide content arises between the low iodide content layer of the central portion and the high iodide content layer on the outside of this, large disturbances are created in the periodic lattice of the crystal with the formation of electron trapping centers, and the photographic speed is reduced.
b) With triple structure silver halide grains as disclosed in JP-A-58-113928, only the intermediate layer has a high iodide content and so the volume fraction of the high iodide content layer with respect to the whole grain is not large. In the case of tabular grains, it is not possible to form seed crystals consisting of 100% tabular grains with nuclei formation alone and so nuclei formation.fwdarw.Ostwald ripening is used, and this results in the size of the seed crystals becoming, in terms of the average grain diameter, from 0.4 to 0.6 .mu.m.
c) Development inhibition in the later stages of a parallel development process is normally effective for improving graininess, but it is difficult to achieve this effect with grains which have a low iodide content in the central portion.
Hence, the production of tabular grains which have a high iodide content in the central portion, which have two parallel twinned crystal planes per grain, which have the characteristics of tabular grains (a large specific surface area), and which have good mono-dispersivity is desirable.